THE FATE OF TRACE-ELEMENTS DURING COAL COMBUSTION AND GASIFICATION - AN OVERVIEW

Coal utilization in combustion or gasification is an important source of trace element emissions to the atmosphere. This paper provides an overview of the fate of trace elements through these processes. The behaviour of trace elements in pollution control systems downstream of the combustor or gasifier is also reviewed. The coal itself is the main source of trace elements in the combustion or gasification system, although some elements are introduced in sorbents, additives or fluxes. During combustion or gasification, coal particles undergo complex changes that lead to the vaporization of volatile elements. Trace elements are partitioned into several output streams, including solid coal residues and flue gases which enter downstream pollution control devices. Elements in the flue gases may be present in the gas phase or bound on particulate aerosols. Gas cleaning devices for SO(x) and NO(x) control may have solid or liquid streams in which some elements from the flue gases are retained and concentrated. Other constituents containing trace elements remain in the flue gases and are emitted to the atmosphere. Because volatile trace elements are enriched in the fine particulate material carried downstream of the combustor or gasifier, the emission of these trace elements depends more on the efficiency of the gas cleaning system than on the method of coal conversion. The effective control of trace element emissions requires an understanding of their path through the various coal conversion processes and downstream pollution control systems.